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What capacitor and inductor are the best for a switching power supply?
Summary of Inductors and Overall Conclusion
Inductor – Part 5
－With the theme of “What capacitor and inductor are the best for a switching power supply”, we have discussed various aspects of capacitors, to start with, and then of inductors. This is our final section, and so we would like to summarize the Inductors edition and end with an overall conclusion to this series.
Series on Capacitors
Series on Inductors
－We begin by summarizing the sections on inductors. We have divided our explanations of inductors into four areas. Let us review the key points of these in order.
In “Part 1: Reading and Understanding Inductor Specifications and Equivalent Circuits”, we explained some of the "implications" that are not made explicit, in order to aid in understanding the specifications and standard values of inductors. For example, an understanding of the meaning of such parameters as the DC bias current is basic; but even when the terms appearing in spec sheets are the same, the specified conditions for the values are not necessarily the same for different products and manufacturers, and so due caution must be exercised when comparing products during selection of inductors.
In addition to understanding the specifications of products, it was also explained that it is important to be familiar with equivalent circuits and the parasitic components of inductors in order to comprehend basic inductor characteristics. These are closely related to specification parameters as well.
－In “Part 2: Features of Various Inductors”, we learned about the types and features of inductors used in switching power supplies.
In recent years, the inductors used in switching power supply circuits have mainly been the wire-wound closed magnetic type, and the multilayer type. Wire-wound closed magnetic type inductors are of two varieties, with a drum sleeve construction and with a sleeveless construction; due to their small size and gradual saturation characteristics, there is a tendency toward more frequent use of sleeveless-structure inductors. Multilayer type inductors were not previously used in switching power supplies, but recently their saturation characteristics and increases in inductance (albeit to only a few μFs or so) have led to use in switching power supplies with oscillation frequencies in the megahertz band. Where size is concerned, they are smaller than wire-wound type inductors, and so are well-suited for power supplies in mobile products.
One matter that should be examined carefully is a problem with mechanical strength of sleeveless-type components due to their structure.. At TAIYO YUDEN, by switching from conventional resins to a resin with high hardness, we have taken measures to improve strength, but strategies are varied among different manufacturers, and it is important that component parameters in this area be studied carefully, for example by obtaining stress testing data relating to mechanical stress and temperature.
－In “Part 3: Matters for Study in Power Supply Circuits”, it was explained how inductor characteristics affect switching power supply circuits.
Here, we explained the DC bias current and the temperature rise current, which are important items requiring study, and indicated their relation to the output current of a step-down converter. We also explained ways of thinking about inductor losses. Quite a bit was discussed, and a number of important points were raised, which can be summarized by the following five items:
- When the maximum allowed DC bias current is exceeded, the inductor saturates and the inductor peak current becomes extremely large, causing a drop in efficiency and anomalous behavior, and in the worst case, destroying the power supply IC.
- If a current larger than the temperature rise current flows, heat generation is increased, and there is the possibility of reduced reliability not only of the inductor, but of nearby components as well. If an unacceptable level of heat generation is reached, burnout may occur as well.
- Inductor power losses are the sum of the DC power loss and the AC power loss; under light loading, the AC loss dominates, but when loading is heavy, it is the DC loss that is dominant.
- In equipment such as smartphones that tend to remain in a wait state for long periods, AC losses are dominant, and when Rac is large, there is the possibility that battery degradation may be accelerated, so caution must be exercised.
- Rac is often not included in tables of standard values. When Rac information is needed, the manufacturer should be consulted.
－Finally, there was a description of how metal inductors have been attracting attention of late.
In “Part 4: Metal Inductors and Trends in Power Supplies”, it was explained that metal inductors are useful for the movement to "low voltages and large currents" that has been one trend in IC power supply voltages in recent years. Metal inductors are well-suited to demands for
lower voltages and larger currents as well as
miniaturization because there have been the following market backgrounds and the performance of metal inductors has been improved in line with the demands of the market.
- In order to supply large currents, a small inductance is necessary.
- For this reason, switching of switching power supplies must be made faster.
- If inductances are smaller, inductor sizes are also smaller.
- Metal inductors have a high DC bias current, and saturation is extremely gradual, and the inductor size can be made smaller than a ferrite inductor with the same DC bias current.
- The metal material of a metal inductor has a low magnetic permeability, and the inductance cannot be made very high, but inductances of about 4.7 μF can be realized in actual use.
When using metal inductors, there are matters that must be confirmed in advance. Metal inductors are extremely small and have excellent saturation characteristics, but there is the issue of degradation of the component insulation at elevated temperatures. Metal inductors are generally called metal-composite type inductors, with an organic resin used to form insulation between iron powder particles. This resin is degraded at high temperatures, causing degradation of the insulation, so that the Q value drops sharply and the efficiency as a power supply falls dramatically, with the further potential problem of heat generation leading to thermal runaway in a vicious circle.
In our case, through development of our own original materials in which individual metal particles each have an insulating oxide film, we offer the MCOIL™ inductor lineup, which resolves the issue of insulation and achieves higher magnetic permeability than metal-composite type inductors, and therefore high inductance.
Finally, with respect to the selective use of metal inductors and ferrite inductors, for inductances up to 5 μF or so, metal inductors should be selected to obtain both their excellent characteristics and advantage of small size. When higher inductances are required, ferrite inductors are the preferred option. Reflecting this situation, the MCOIL lineup currently offers the inductances up to 4.7 μF.
－As is the case with capacitors, there is a wealth of materials and information that cannot be obtained from websites, as well as many essential points to consider, and I personally was impressed with the depth of knowledge involved in understanding capacitors and inductors. Could we please have an overall summary, including capacitors as well as inductors.
It pleases me greatly to hear this. Overall, what I as an engineer would like to say is that, while it is important to thoroughly comprehend data sheet specifications and standard values, I think it is also vital to have a thorough understanding of the basic current, voltage, and temperature characteristics that underlie those specifications, as well as the parasitic components that we explained using equivalent circuits. If these matters can be grasped and witnessed firsthand, debugging and troubleshooting in the design stage should be possible. At TAIYO YUDEN, various technical support is available, and I would hope that you will consult us when you run into difficulties.
－Thank you very much for all your time.
What capacitor and inductor are the best for a switching power supply? －Part 1－
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What capacitor and inductor are the best for a switching power supply? －Part 2－
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What capacitor and inductor are the best for a switching power supply? －Part 3－
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What capacitor and inductor are the best for a switching power supply? －Part 4－
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